1. Field of the Invention
The present invention relates to a throttle valve control system for an internal combustion engine, and particularly to a throttle valve control system which suppresses a rapid opening of a throttle valve of the engine immediately after a cold start of the engine.
2. Description of the Related Art
Japanese Utility Model Publication No. H3-13541 (JU-'541) discloses a throttle control system which sets a valve opening speed of a throttle valve according to a warm-up condition of the engine. According to this system, when an engine coolant temperature is less than a predetermined temperature, the valve opening speed is regulated to a value equal to or less than a maximum speed corresponding to the engine coolant temperature. The regulation of the valve opening speed prevents the throttle valve from rapidly opening immediately after the cold start of the engine, thereby preventing an air-fuel ratio of the air-fuel mixture to be supplied to the engine from becoming lean.
Further, Japanese Patent Publication No. 3044719 (JP-'719) discloses a method for controlling the valve opening speed of the throttle valve according to an alcohol concentration in the fuel containing alcohol. According to this method, when calculating the throttle valve opening speed from a depressing speed of an accelerator, the throttle valve opening speed is controlled so as to become lower as the alcohol concentration increases.
A temperature in the combustion chamber of the engine, which most significantly affects the combustion state of the engine, increases as an elapsed time period from the engine start (from the time the self-sustaining operation of the engine starts) becomes longer. However, this point is not taken into consideration in the system of JU-'542. That is, when the temperature in the combustion chamber increases, there is a delay until the engine coolant temperature increases. Therefore, the increasing speed of the throttle valve opening is suppressed when the engine coolant temperature is low, and the drivability of the engine may deteriorate.
Further, the method shown in JP-'719 aims to maintain a degree of acceleration corresponding to the accelerator operation regardless of the alcohol concentration. Therefore, JP-'719 does not provide a method for preventing deterioration of the combustion state immediately after the engine start.